1. Field of the Invention
This invention relates to a semiconductor substrate and a semiconductor device especially suitable for use in semiconductor lasers using nitride III-V compound semiconductors, such gallium nitride (GaN).
2. Description of the Related Art
Active researches. have been made in recently years on semiconductor lasers using nitride III-V compound semiconductors having wurtzite-structured crystal structures, represented by GaN as semiconductor lasers.
When a semiconductor laser using nitride III-V compound semiconductors is fabricated, GaN, AlGaN, GaInN, and so on, are epitaxially grown in multiple layers on a sapphire substrate or a silicon carbonate (SiC) substrate to make a structure sandwiching an active layer between an n-type cladding layer and a p-type cladding layer. Good-quality epitaxial layers can be made when {0001}-oriented surfaces of these layers results in being parallel to the substrate when they are grown by the epitaxial growth. There is a report on continuous oscillation at a room temperature with a semiconductor laser made by epitaxial growth (IEEE Lasers and Electro-Optics Society 1996 Annual Meeting, Postdeadline Papers, Nov. 1996).
When a semiconductor laser is fabricated, it is necessary to make an edge used as a reflective surface of the cavity. The cavity edge is usually made by cleaving a structure having epitaxially grown layers on a substrate along a face normal to the layer surfaces or interfaces, utilizing the cleavability of crystals. In this case, the cleavability of the substrate much thicker than epitaxial layers is especially important. In case of semiconductor lasers using III-V compound semiconductors such as AlGaAs, AlGaInP, and so forth, and semiconductor lasers using II-VI compound semiconductors such as ZnSSe, CdZnSe, and so forth, by utilizing a high cleavability of a GaAs substrate used as the substrate and by cleaving epitaxial layers, also having a high cleavability, together with the GaAs substrate, flat cavity edges normal to the substrate surface or surfaces of the epitaxial layers are formed.
There are other methods used for making cavity edges normal to substrate surfaces, such as reactive ion etching (RIE) or chemical wet etching. However, since nitride III-V compound semiconductor crystals exhibit a high chemical stability, it is very difficult to actually make cavity edges by using chemical wet etching. If RIE is used, damages to cavity edges or unacceptable flatness of cavity edges arise as problems.
Therefore, in semiconductor lasers using nitride III-V compound semiconductors, it has been tried to make cavity edges by cleavage. Also for laser semiconductor lasers using low-cleavable substrates, such as sapphire substrates, on which laser oscillation has been reported, epitaxial layers grown on a sapphire substrate were cleaved simultaneously by cleaving the sapphire substrate. Usually, such sapphire substrates have {0001}-oriented major surfaces, and cavity edges are {11-20}-oriented surfaces of epitaxial layers forming laser structures. A reason why sapphire substrates having {0001}-oriented major surfaces are used lies in that {11-20}-oriented surfaces of crystals having wurtzite-structured crystal structures have atomic arrangements (bonding) similar to those of {110}-oriented surfaces which are cleavage surfaces of crystals having zinc blende structures similar to wurtzite-structured crystal structures, and have a cleavability to a certain extent.
However, cavity edges actually made by the method were found to exhibit uneven faces with longitudinal ridge-shaped steps approximately normal to the substrate surfaces. They were probably caused by a low cleavability of {11-20}-oriented surfaces, stresses in epitaxial layers (due to a difference in lattice constant with substrates), and crystallographic irregularities in epitaxial layers (presence of internal crystallographic defects or mosaic crystals). Cavity edges having such unevenness are bad in reflective efficiency and parallelism, and fail to exhibit acceptable characteristics as cavities. These are factors that not only degrade characteristics and efficiencies of semiconductor lasers, but also cause deterioration of devices and degradation of their reliability.
There is another trial of growing a thick GaN crystal layer on a sapphire or other substrate and using the GaN crystal as a substrate to epitaxially grow nitride compound III-V semiconductors thereon. This method also uses {0001}-oriented surface to epitaxially grow nitride III-V compound semiconductors layers, and result in using {11-20}-oriented surfaces as cleavage surfaces like the method explained above. Here again, therefore, it is difficult to obtain good-quality cavity edges.
As discussed above, it has been difficult to obtain good-quality cavity edges by cleavage in semiconductor lasers using nitride III-V compound semiconductors.
It is therefore an object of the invention to provide a semiconductor substrate making it easy to obtain flat and optically excellent cleavage surfaces and hence suitable for use in fabrication of semiconductor lasers using nitride III-V compound semiconductors, and to provide a semiconductor device using the semiconductor substrate, having a high efficiency, long life and high reliability, and suitable for use in semiconductor lasers.
According to the invention, there is provided a semiconductor substrate made of a nitride III-V compound semiconductor having a wurtzite-structured crystal structure, and having a major surface normal to the {0001}-oriented face.
According to the invention, there is also provided a semiconductor device comprising: a semiconductor substrate made of a nitride III-V compound semiconductor having a wurtzite-structured crystal structure, and having a major surface normal to the {0001}-oriented face; and nitride III-V compound semiconductor layers on said semiconductor substrate.
In the present invention, the major surface of the semiconductor substrate may be any of faces normal to the {0001}-oriented face (called C-face). Specifically, a {01-10}-oriented face (called M-face), {11-20}-oriented face (called A-face), or other faces slightly offset from these faces, may be used. When a face offset from the {01-10}-oriented face or the {11-20}-oriented face is used as the major surface of the semiconductor substrate, the major surface, preferably, is substantially normal to the {0001}-oriented face with an offset angle, if any, within xc2x15xc2x0.
Throughout the present application, {01{overscore (1)}0} is expressed as {01-10}, and {11{overscore (2)}0} as {11-20}, to describe surface orientations.
In the present invention, nitride III-V compound semiconductor layers contain at least one of group III elements selected from the group consisting of Ga, Al, In and B, and at least N, and may additionally contain group V elements involving As or P. Specific examples of the nitride III-V compound semiconductor layers are a GaN, layer, AlGaN layer, GaInN layer, AlGaInN layer, and so forth.
The semiconductor substrate according to the invention can-be manufactured easily by the method explained below, for example. That is, first grown is a bulk crystal of a nitride III-V compound semiconductor having a wurtzite-structured crystal structure. The nitride III-V compound semiconductor bulk crystal can be grown, for example, by growth from liquid Ga at a high temperature of 1400xc2x0 C. through 1600xc2x0 C. in a high-pressurized nitrogen atmosphere of 10 through 20 kbar (J. Crystal Growth 166(1996) 583) or by a flux method or sublimation configured to promote growth by heating metal Ga and sodium azide confined in a stainless tube to 600 through 700xc2x0 C. Then, the nitride III-V compound semiconductor bulk crystal is cut along a face substantially normal to its {0001}-oriented face, namely, {01-10}-oriented face (M-face) or {11-20}-oriented face (A-face), for example, and the surface of the cut-out crystal is polished to smooth the surface. As a result, the semiconductor substrate according to the invention can be obtained.
Alternatively, the semiconductor substrate according to the invention can be manufactured by another method explained below. That is, epitaxially grown is a bulk crystal of a nitride III-V compound semiconductor with a sufficient thickness, for example, as thick as 100 xcexcm or more, on a crystal substrate such as sapphire substrate or SiC substrate, by sublimation or hydride vapor phase epitaxy (HVPE), for example, so that a surface substantially normal to its {0001}-oriented face, namely, the {01-10}-oriented surface or the {11-20}-oriented surface, for example, be parallel to the substrate. Then, the surface of the epitaxial layer is polished to smooth the surface. After that, by removing the substrate used for the growth from the bottom mechanically or by chemical wet etching, the semiconductor substrate according to the invention can be made. Prior to removal of the sapphire substrate or SiC substrate, one or more nitride III-V compound semiconductor layers may be epitaxially grown on the nitride III-V compound semiconductor bulk crystal epitaxially grown on the substrate, and the substrate may be removed thereafter.
According to the invention having the above-summarized construction, the semiconductor substrate made of a nitride III-V compound semiconductor having a wurtzite-structured crystal structure and having a major surface substantially normal to its {0001}-oriented face can be readily cleaved along the {0001}-oriented face because the {0001}-oriented face is the face having the highest cleavability in a crystal having the layered-structured wurtzite-structured crystal structure, namely, the high-cleavable face. Therefore, when nitride III-V compound semiconductor layers are epitaxially grown on the semiconductor substrate to for a semiconductor laser, cavity edges can be formed easily by cleaving these nitride III-V compound semiconductor layers together with the semiconductor substrate along {0001}-oriented faces.
Moreover, since the semiconductor substrate is made of a nitride III-V compound semiconductors, both the semiconductor substrate and the nitride III-V compound semiconductor layers thereon have wurtzite-structured crystal structures, and coincide, or are very near, in lattice constant. Therefore, the nitride III-V compound semiconductor layers have a good quality, remarkably diminished in stress and inner crystallographic defects. Additionally, since the semiconductor substrates and the nitride III-V compound semiconductor layers are substantially equal in lattice constant, the nitride III-V compound semiconductor layers can be grown epitaxially, without the need for a buffer layer of GaN or AlN grown under a low temperature prior to the epitaxial growth of the nitride III-V compound semiconductor layers, which is required when using a sapphire substrate, for example. Therefore, the present invention can prevent a mosaic structure from being produced in the nitride III-V compound semiconductor layers, which must be taken into account when using a sapphire substrate, for example. Thus, the nitride III-V compound semiconductor layers have an excellent quality also in this respect.
For these reasons, cleavage surfaces obtained by cleaving the semiconductor substrate together with the nitride III-V compound semiconductor layers thereon exhibit an optically excellent quality with substantially no unevenness, and are suitable for use as cavity edges of semiconductor lasers.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.